AttributeError while changing the display units using python script
Error messages are pretty helpful in this case. 


  "AttributeError: 'Scene' object has no attribute 'system'"


Means that you can not access the scene's system property because there is none. In other words: the actual 'Object' (Scene in this case) has no attribute called .system so you can not access Scene.system. 

Suggest using the auto complete feature (Ctrlspacebar) of the command line when searching for attributes:

&gt;&gt;&gt; bpy.context.scene.u
                       nit_settings
                       pdate(
                       sers
                       ...
&gt;&gt;&gt; bpy.context.scene.unit_settings.system
'METRIC'
&gt;&gt;&gt; bpy.context.scene.unit_settings.scale_length
1.0


Note: Lines from the Info Area not always yield the 'correct path' to the attribute/property.